Conventionally, there have been proposed an apparatus that conducts a heavy charged particle therapy by radiating charged particles to an affected area such as cancer cells. In a heavy charged particle therapy such as a carbon filament therapy, it is desirable to realize a uniform clinical effect in the target. For achieving this, it is possible to define a clinical dose which is the product of an absorbed dose and a relative biological effectiveness (RBE), and make an irradiation plan so that the clinical dose is uniform in the target.
FIG. 4(A) is a diagram illustrating irradiation spots to which charged particles of a single ion species are to be radiated. FIG. 4(A) shows a longitudinal section of a target viewed from the lateral side of the traveling direction of the beam. In the apparatus that conducts a heavy charged particle therapy, as illustrated in the drawing, spots SP disposed on the surface perpendicular to the irradiation direction are arranged in the irradiation direction with respect to a tumor region 182 located behind a body surface 188, and thus the spots SP are arranged three-dimensionally. The apparatus that conducts a heavy charged particle therapy sequentially radiates a beam of ion species to the spots SP from the direction of the arrow illustrated in the drawing and conducts irradiation in a manner of filling the tumor region 182.
FIG. 4(B) illustrates a depth dose distribution chart by such a carbon filament therapy. In this chart, depth distributions of a clinical dose 191, an RBE 192, and an ion species irradiation dose 193 are shown. In designing a clinical dose that is uniform in the target in the carbon filament therapy, the quality (LET) distribution of carbon filament for realizing this is determined almost uniquely. Here, when the RBE 192 involves errors 192a, 192b, large distortions 191a, 191b occur in the distribution of the clinical dose 191, and the clinical dose distribution can greatly deteriorate.
RBE depends on quality of radiation (particle species or LET), dose level, cell strain, end point and so on, and RBE itself is accompanied by a large error. Therefore, it is desired to reduce the error in RBE for preventing significant deterioration in the clinical dose distribution.
There have been proposed a method and an apparatus for charged particle beam irradiation capable of radiating charged particles from a plurality of directions by having a rotary irradiation device (see Patent Document 1). With this apparatus, since charged particles can be radiated from the plurality of directions, it is possible to reduce the irradiation dose on normal sites by widely dispersing the dose to be radiated to the normal sites. Radiation of the charged particles from the plurality of directions can also reduce an error in RBE.
Increased irradiation directions, however, lead to several disadvantages. First, increased irradiation directions disadvantageously increase a burden on a staff engaged in the therapy. In addition, increased irradiation directions disadvantageously lead to a large increase in exposure volume of normal tissues. There is a disadvantage that a rotary gantry like a rotary irradiation device of Patent Document 1 is bulky. Also, there is a disadvantage that a rotary gantry for heavy charged particles has not been practically used in an actual therapy because of difficulties in its construction and operation.
Besides the above, since occurrence of a delayed effect such as cerebral necrosis from the planned therapeutic volume (PTV) after the therapy is reported for part of sites such as a cerebral tumor, it is desired to develop an irradiation method capable of effectively controlling only cancer cells without injuring normal cells contained in a tumor.